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MSC3267: reference relationships

Proposal

This proposal defines a relation type (using MSC2674 relations) for events to make a reference to another event.

A rel_type of m.reference is defined as a generic way to associate an event with another event. As a bundle, m.reference relations appear as an object with a single chunk field. The chunk is an array of objects with a single event_id field for all the child events which m.reference the parent.

There are no implied semantics by a reference relation: the feature or event type which makes use of the rel_type should specify what sort of semantic behaviour there is, if any. For example, describing that a poll response event references the poll start event, or that a location update references a previous location update.

Reference relations are used by MSC2241 to tie all events together for the same verification request.

For instance, an m.room.message which references an existing event would look like:

{
    // Unimportant fields omitted
    "type": "m.room.message",
    "content": {
        "msgtype": "m.text",
        "body": "i <3 shelties",
        "m.relates_to": {
            "rel_type": "m.reference",
            "event_id": "$another_event_id"
        }
    }
}

Server aggregation

MSC2674 states that values for rel_type should define how the server should aggregate the rel_type. For m.reference, child events are bundled to appear as follows under m.relations:

{
  "m.reference": {
    "chunk": [
      {"event_id": "$one"},
      {"event_id": "$two"},
      {"event_id": "$three"}
    ]
  }
}

Note that currently only the event_id is noted in the chunk, however a future MSC might add more fields.

Limitations

Different subtypes of references could be defined through additional fields on the m.relates_to object, to distinguish between other forms of semantic behaviour independent of type (hypothetical threads, replies, etc if we didn't have a system for them already). This MSC doesn't attempt to define these subtypes.

This relation cannot be used in conjunction with another relation due to rel_type being a single value. This is known and unfortunately not resolved by this MSC. A future MSC might address the concern.

Edge Cases

Can you reference an event which doesn't have logical meaning? Eg to a reaction?

Yes, at the protocol level. But you shouldn't expect clients to do anything useful with it.
The relationship is effectively pointless, so the event would appear as though there was no reference relationship.

Do we need to support retrospective references?

For something like "m.duplicate" to retrospectively declare that one event dupes another, we might need to bundle two-levels deep (subject+ref and then ref+target). We can cross this bridge when we get there though, as another aggregation type

Unstable prefix

Unfortunately not applicable - this MSC was used in production and appears in the specified version of the key verification framework.

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